This invention relates to a heat-resistant (ferritic) cast steel having excellent thermal fatigue resistance, which is used mainly to form a part subjected to repeated heating and cooling, such as a part of an automobile engine.
Recently, from the viewpoints of thermal fatigue resistance and economy, heat-resistant ferritic cast steel has been used as a material for an automobile engine part such as a precombustion chamber of a diesel engine, and various kinds of such cast steel have been proposed.
For example, Japanese Patent Examined Publication No. 46-18845 proposes a heat-resistant ferritic steel which is intended mainly to achieve improved deformation resistance and crank resistance, and consists of 0.05 to 0.40% C, 0.5to 1.0% Si, 0.2 to 1.0% Mn, 20.0 to 23.0% Cr, 0.5 to 2.5% Mo, 0.5 to 3.5% W, 0.5 to 3.5% Nb and the balance Fe and incidental impurities. Japanese Patent Unexamined Publication No. 48-52618 proposes a heat-resistant ferritic steel which has the same composition as that of the steel proposed in the above Japanese Patent Examined Publication No. 46-18845 except that instead of the W content, 1.0 to 4.0% Ni is contained in the steel in order to improve toughness and oxidation resistance. Japanese Patent Examined Publication No. 54-18647 proposes a heat-resistant ferritic cast steel which is obtained by containing 0.01 to 0.15% B and 0.01 to 0.15% Zr in the steel proposed in the above Japanese patent Unexamined Publication No. 48-52618. This heat-resistant ferritic cast steel of Japanese Patent Examined Publication No. 54-18647 exhibits a satisfactory crack resistance even when this steel is used in an engine subjected to a severer thermal load.
In order to mainly improve heat crack resistance and oxidation resistance, Japanese Patent Unexamined Publication 56-41354 proposes a heat-resistant ferritic steel consisting of 0.1 to 0.5% C, not more than 3.5% Si, not more than 2.0% Mn, not more than 12.0% Ni, 20 to 30% Cr and the balance Fe and incidental impurities, such a heat-resistant ferritic steel further containing a predetermined amount of at least one kind selected from the group consisting of Mo, W, Nb, V and Ti, such a heat-resistant ferritic steel further containing a predetermined amount of at least one kind selected from the group consisting of Cu, Co, B and R.E (rare earth element), and such a heat-resistant ferritic steel further containing a predetermined amount of S.
Japanese Patent Examined Publication No. 62-17021 proposes a heat-resistant ferritic steel which is inexpensive because Ni is not added, and has excellent crack resistance, and consists of 0.05 to 0.4% C, 0.05 to 2.0% Si, 0.05 to 2.0% Mn, 18.0 to 25.0% Cr, 0.01 to 0.50% Nb and the balance Fe and incidental impurities.
Japanese Patent Unexamined Publication No. 61-117251 proposes a heat-resistant ferritic steel which is intended mainly to achieve improved thermal fatigue resistance and consists of 0.05 to 0.40% C+N, 0.5 to 3.5% Si, not more than 2.0% Mn, 18.0 to 25.0% Cr, 0.2 to 2.0% Al, 0.1 to 1.5% of at least one kind selected from the group consisting of Nb, Ti and Zr, and the balance Fe and incidental impurities, and such a heat-resistant ferritic steel further containing a predetermined amount of at least one kind selected from the group consisting of Ni, Mo, W, V and B.
However, recently, in order to improve the performance of an engine, the temperature at which the engine part is used tends to become higher, and it has now been desired to provide heat-resistant cast steel of the type which is more excellent in thermal fatigue resistance and less costly than the heretofore-proposed heat-resistant steels.